The standardization of LTE-A (Long Term Evolution-Advanced) is currently under discussion in 3GPP (3rd Generation Partnership Project) as a next generation communication standard for LTE (Long Term Evolution). A carrier aggregation (CA) technology is introduced in an LTE-A system to maintain backward compatibility with an LTE system and to achieve higher throughput than the LTE system. According to the carrier aggregation technology, an LTE carrier (also referred to as a “component carrier”) having a maximum bandwidth of 20 MHz, which is supported by the LTE system, is used as a basic component. Then, a plurality of component carriers are simultaneously used, thereby achieving broader bandwidth communication.
According to the carrier aggregation, an arrangement of component carriers as shown in FIG. 1 is assumed. In Case 1, two adjacent 20 MHz bandwidths can be used as a 40 MHz bandwidth according to the carrier aggregation. Case 1 can be effectively used when a continuous bandwidth broader than 20 MHz is provided, for example, and can maintain backward compatibility with an LTE system and achieve higher throughput than the LTE system. In case 2, two separate 10 MHz bandwidth can be used as a 20 MHz bandwidth according to the carrier aggregation. Case 2 can be effectively used when frequency allocation to an operator is discontiguous, for example, and can maintain backward compatibility with an LTE system and achieve higher throughput than the LTE system.
According to the carrier aggregation, a mobile station (UE: User Equipment) can communicate with a base station (eNB: evolved Node B), simultaneously using a plurality of component carriers. In the carrier aggregation, a primary cell (PCell: Primary Cell) and a secondary cell (SCell: Secondary Cell) are configured. The primary cell is reliable and used to maintain connectivity with a mobile station. The secondary cell is additionally configured for a mobile station connecting the primary cell. For example, in the cases shown in FIG. 1, one of the two component carriers may be configured as a primary cell and the other may be configured as a secondary cell (see 3GPP TS 36.300 V11.3.0 (2012-09), section 7.5).
A primary cell is a cell configured in a similar manner to an LTE system and a cell to maintain connectivity between a mobile station and a network. Thus, a mobile station can receive a PDCCH (Physical Downlink Control Channel) and a PDSCH (Physical Downlink Shared Channel) and transmit a PUCCH (Physical Uplink Control Channel), a PUSCH (Physical Uplink Shared Channel), and a PRACH (Physical Random Access Channel) in a primary cell. When a primary cell is changed, a mobile station needs to perform handover.
On the other hand, a secondary cell is a cell which is configured for a mobile station in addition to the primary cell. An addition and a removal of a secondary cell are performed using an RRC (Radio Resource Control) configuration. A mobile station does not transmit a PUCCH and a PRACH in a secondary cell. A secondary cell is in a deactivation state, immediately after the secondary cell is configured for a mobile station. Communication or scheduling in the secondary cell is possible only after the secondary cell is activated in a MAC (Medium Access Control) layer. Thus, in order to allow communication in the secondary cell, in other words, in order to allow scheduling in the secondary cell, the secondary cell needs to be transitioned to an activation state.
In an LTE-A system currently under discussion, in order to transition a secondary cell to the activation state or the deactivation state, a base station transmits a MAC CE (control element) in the MAC layer to a mobile station to instruct the mobile station to activate/deactivate the configured secondary cell, as shown in FIG. 2. When the mobile station receive the explicit instruction of the state transition, the mobile station transitions the secondary cell to the instructed state.
In addition, when the mobile station receives from the base station a MAC CE for activating the secondary cell, the mobile station activates the secondary cell and starts an SCell Deactivation Timer to transition the secondary cell to the deactivation state after a predetermined time period. The base station also starts an SCell Deactivation Timer when the base station transmits the MAC CE or when the base station receives an acknowledgement (ACK) from the mobile station. In other words, each of the base station and the mobile station maintains its own SCell Deactivation Timer and manages the state of the activated secondary cell. After the SCell Deactivation Timer is started, when the base station performs scheduling of new radio resources for the mobile station in the secondary cell, each of the base station and the mobile station restarts its own SCell Deactivation Time. Then, when the SCell Deactivation Timer expires or when the mobile station receives an explicit instruction of the state transition to deactivate the secondary cell, the mobile station transitions the secondary cell to the deactivation state. Also, when the SCell Deactivation Timer for the secondary cell expires or when the base station receives a notification from the mobile station that the secondary cell is deactivated, the base station determines that the secondary cell is deactivated and stops scheduling of radio resources for the mobile station in the secondary cell. In this manner, consistency of the activation/deactivation state of the secondary cell is maintained between the base station and the mobile station.